Piezo-electric crystal circuit arrangements



26, 1969 D.J.YFEWINGS 3,463,945

PIEZO-ELECTRIC CRYSTAL CIRCUIT ARRANGEMENTS Filed Jan. 23, 1967 I 2Sheets-Sheet 1 |NVE NTOR Jar film/m WWd/QZBMM W ATTORNEYS Aug. 26. 1969D. J. FEWINGS 3,463,945

PIEZO-ELECTRIC CRYSTAL CIRCUIT ARRANGEMENTS Filed Jan. 23, 1967 2Sheets-Sheet 2 .BY A M/40W A [i/www ATTORNEYS United States Patent3,463,945 PlEZO-ELECTRIC CRYSTAL CIRCUIT ARRANGEMENTS David JohnFewings, Essex, England, assignor to The Marconi Company Limited,Strand, London, England Filed Jan. 23, 1967, Ser. No. 610,870 Claimspriority, application Great Britain, Jan. 28, 1966, 3,944/66; Sept. 30,1966, 43,834/66 Int. Cl. H03!) 5/32 U.S. Cl. 3108.9 5 Claims ABSTRACT OFTHE DISCLOSURE Piezo-electric circuit arrangement of good temperaturestability consisting of a number of crystals each having a parabolicfrequency/temperature characteristic and each connected in one of a likenumber of paralleled circuits. The crystals are chosen to have theirindividual turnover temperatures at the same frequency but at differenttemperatures spaced apart over a temperature range to be covered.

This invention relates to piezo-electric crystal circuit arrangementsand has for its object to provide improved multiple crystal circuitarrangements in which the variation of frequency with temperature shallbe relatively small and very small in relation to thefrequency-temperature characteristic which would be exhibited by one ofthe crystals of the arrangement if employed alone. The invention, thoughnot limited to such use, is partcularly well adapted for use inconjunction with the invention contained in the co-pending applicationof David John Fewings and Harvey Joyce Pipe, Ser. No. 539,059, filedMar. 31, 1966.

Certain forms of crystal, notably AT cut crystals, exhibit onlycomparatively small frequency variations over a wide temperature rangebut such crystals have the serious defects that the law relatingfrequency with temperature is complex and not predictable in advance ofmanufacture. This means that, in many cases, when a crystal is requiredfor a particular oscillator it has to be selected from a batch.Moreover, if a compensating circuit (e.g. as in the aforesaid co-pendingapplication No. 539,059) for compensating for temperature variations offrequency is employed for such a crystal it becomes complicated anddifficult to design.

There are, however, forms of crystal which exhibit a low orcharacteristic connecting frequency (ordinates) with temperature(abscissae) which is substantially parabolic in shape and has what iscalled a turnover temperature i.e. a temperature, corresponding to thecrown of the parabola, where maximum frequency occurs. Thesesubstantially parabolic characteristic crystals are considerably betteras regards complexity of the law connecting frequency with temperatureand, moreover, individual such crystals taken from a batch are much morealike in performance. Such crystals, however, are considerably morefrequency-dependent on temperature, especially when the temperaturerange is wide. Because of the substantially parabolic shape of thecharacteristic a low rate of change of frequency with temperature isobtained only over a limited temperature range around the turnovertemperature. Outside this limited range frequency changes increasinglyrapidly with change of temperature. Moreover if a compensating circuitis required to be used in conjunction with such a crystal it becomesundesirably complex and expensive if the range over which it is requiredto provide compensation is materially wider than the limited range abovementioned. To quote practical figures a compensating circuit asdescribed in the aforesaid co-pending application Ser. No. 539,059 ifused with a BT cut crystal (a well known form of crystal having asubstantially parabolic frequency-temperature coefficient) having aturnover temperature of 25 becomes considerably more complex andexpensive and requires more transistors if the temperature rangematerially exceels 0-50 than if that range is all that has to behandled.

The present invention seeks to provide improved piezoelectric crystalcircuit arrangements which shall, despite employing crystals each ofwhich individually has a substantially parabolic frequency-temperaturecharacteristic, exhibit a wide range frequency-temperaturecharacteristic with considerably less variation in frequency than doesthe characteristic of an individual component crystal.

According to this invention a piezo-electric crystal circuit arrangementcomprises a plurality of crystals each in one of a plurality or circuitsin parallel and each having an at least approximately parabolicfrequency-temperature characteristic said crystals having theirindividual tumover temperatures occurring at substantially the samefrequency but at different temperatures spaced apart in a temperaturerange to be covered.

In the preferred and simplest embodiment of the invention there are twocircuits in parallel, one containing one crystal and the othercontaining a second crystal in series with a reactance dimensioned tocause the second crystal to have its turnover temperature at the samefrequency as that of the first despite being at a dilferent temperature.The invention is not, however, limited to the use of only two crystalsin parallel circuits and larger numbers of crystals in parallelcircuits: for example five crystals in parallel circuits have beensuccessfully employed in experimental practice.

One form of crystal which may be employed in carrying out the inventionis the BT cut crystal.

The invention is illustrated in the accompanying drawings in whichFIGURE 1 is an explanatory graphical figure and FIGURE 2 is a circuitdiagram of one embodiment of the invention.

Referring to FIGURE 1, curves A and B are the frequency-temperaturecharacteristics of two BT cut crystals A and B, of which A, alone and asmanufactured, has a turnover temperature of 27 C. and B, alone and asmanufactured, has a turnover temperature of C. In FIGURE 1 F isfrequency and T is temperature. These two crystals are connected in twoparallel circuits, a preferred oscillator circuit arrangement using .atransistor as the active element being shown in FIGURE 2. As will beseen each of these two circuits includes a resistance shunted adjustableinductance L1 or L2 and condensers C1 and C2, which may also beadjustable if desired, are connected as shown. The values of condensersC1 and C2 are chosen to secure maximum flatness of thefrequencytemperature characteristic of the combination and theinductances L1 and L2 enable the turn-over frequency of each crystal tobe set at the required value despite minor variations due to unavoidablycutting tolerances. The circuit is so dimensioned that the curve B islifted along the frequency axis to curve B the turnover temperature ofwhich is substantially at the same frequency as that of curve A. CurvesA and B are substantially symmetrical about the middle of thetemperature range, i.e. about the temperature of 51 C. Curves D1, D2typify resultant frequency-temperature characteristics obtainable fromarrangements of the nature of that shown in FIGURE 2.

The actual curve obtained in any individual case depends on detaileddesign. The curve D1, which mathematical analysis indicates to be themaximally flat curve, is obtainable by suitable choice of componentvalues, but, depending on the component values actually chosen, any of afamily of curves lying approximately between curves D1 and D2 e.g. curveD3, can be obtained. It will be seen that in none of the curves is thereany great variation of frequency with temperature and, in curve D1, thefrequency varies very little with temperature. The frequencies of theseoverall resultant curves are higher than those of either curve A or Bbecause of modification of circuit parameters due to the presence ofboth crystals. It may be shown mathematically that if an oscillator isoffered two alternative frequencies at which it can oscillate it willoscillate at the frequency at which the series resistance is lower. Inthe embodiment of FIGURE 2 there are in effect two series resonantcircuits in parallel and at any given temperature the oscillatorfrequency tends to be that of the crystal offering the lower seriesresistance at that temperature, i.e. the crystal which at thattemperature resonates at the higher frequency. In the middle temperaturerange (around 51 C.) both crystals offer substantially equal seriesresistances and both contribute to the oscillator frequency. In apractical embodiment the changeover from one crystal to the other as thetemperature moves through the mid-temperature occurs smoothly and nodiscontinuity is discernable in practice.

The condition to be satisfied to obtain maximum fiatness of thefrequency-temperature characteristic is X =81rL}3T (1) where X =to thereactance of the total circuit capacitance in series with the crystalcombination (i.e. the reactance of C and C in series in FIGURE 2 L=theinductance of each crystal (assuming they are equal in inductance) B=afwhere at is the coefficient (see Equation 2 below) of either parabola(assuming they are equal and f is the frequency of either crystal atturnover and T =half the difference in temperature between the twoturnover temperatures. Because the frequency-temperature coefficient is(theoretically) parabolic where:

=frequency at the turnover temperature T and f=frequency at anytemperature T Equation 1 above enables crystals to be chosen to providemaximum compensation at any desired frequency. Equation 3 belowdetermines the frequency of the crystal combination and enables crystalsto be chosen to give a desired compensated frequency f As will be seenfrom Equation 1 the limit of compensation is reached when X becomes toolarge for the oscillator to oscillate. The possible range ofcompensation at any fre quency is determined by the crystal inductanceat that frequency. BT cut crystals have relatively low inductance at 8mc./s. and therefore this is a favourable frequency as regardscompensation. It has been found possible, in experimental practice atthis frequency and using only two BT cut crystals in parallel circuits,to keep the frequency constant to one part in a million over atemperature range extending from 26 C. to +66 C. Even wider temperatureranges can be handled by increasing the number of crystals. Calculationindicates that at 10.5 mc./s., it is possible to achieve compensation toone part in a million over a range of 40 C. to +80 C. by using threecrystals in parallel circuits. Any small remaining variations offrequency with temperature can easily be virtually eliminated, ifrequired, by using a compensating circuit, for example, as described inthe aforesaid co-pending application Ser. No. 539,059 despite that awide range of temperature variation may be involved.

The invention is, of course, not limited to the particular circuit shownin FIGURE 2 and other (and simpler) circuits are possible. Pribabaly thesimplest circuit consists merely of two branches in parallel, onecontaining one crystal alone and the other containing a second crystalin series with a condenser.

I claim:

1. A piezo-electric crystal circuit arrangement comprising a pluralityof crystals each in one of a plurality of circuits in parallel, each ofsaid crystals having an at least approximately parabolicfrequency-temperature characteristic, said crystals having theirindividual turnover temperature occurring at substantially the samefrequency but at different temperatures spaced apart in a giventemperature range to be covered and the frequencytemperaturecharacteristic of each of said crystals intersecting thefrequency-temperature characteristics of those of said crystals havingadjacent turnover temperatures.

2. An arrangement as claimed in claim 1 wherein there are two circuitsin parallel, one containing one crystal and the other containing asecond crystal in series with a reactance dimensioned to cause thesecond crystal to have its turnover temperature at the same frequency asthat of the first despite being at a different temperature.

3. An arrangement as claimed in claim 1 wherein each crystal has anadjustment inductance in series therewith.

4. An arrangement as claimed in claim 1 wherein the crystals are ET cutcrystals.

5. An arrangement as claimed in claim 1 wherein the circuit elements aredimensioned at least aprpoximately to satisfy the equation x STrLQT asherein defined.

References Cited UNITED STATES PATENTS 3,152,295 10/1964 Schebler 3l08.1 3,175,168 3/1965 Miyake 3l0--8.1 3,181,045 4/ 1965 Bruntil 310-813,243,726 3/1966 Aemmer 331116 3,260,960 7/1966 Bangert 331116 3,289,07611/1966 Longuemare 3311 16 3,297,961 1/1967 Frerking 331116 3,322,9815/1967 Brenig 310--8.1 3,349,348 10/1967 Ice 3108.l 3,370,255 2/1968Brower 331-416 J D MILLER, Primary Examiner

